First time posting on this board, and just had a question regarding membrane formation. Why is it that things like phosphlipids and glycosphingolipids (like sphingomyelin and glycosphingolipids) form lipid bilayers when theyre exposed to water, whereas some things form micelles instead? i know that the structure of these is rectangular, but what exactly causes them to form the linear side-by-side bilayers, instead of just curling up circularly like a micelle? They both seem to interact similarly with water too (expose hydrophilic polar heads but hide their hydrophobic tailes on the inside). My notes don't do a very good job of explaining this, so any sort of insight would be greatly appreciated. Thanks!

Yeah, this is an idea that seems to get glossed over some of the time. It seems the natural tendency of molecules with hydrophilic heads / hydrophobic tails is to form micelles, because this is the simplest structure that maximizes hydrophobic/philic interaction. In phospholipids, however, the hydrophobic tails have a greater cross-sectional area than the heads, and so they cannot form stable micelles - in other words, the volume of the interior of a hypothetical phospholipid micelle would not allow the hydrophilic heads to make a continuous, unbroken surface. The simplest way for phospholipids to "hide" their hydrophobic tails is the lipid bilayer configuration. Hopefully that helps anyone who stumbles across this board in the future.